The two main engine exhausts and (smaller) auxilliary power unit exhaust. During startup procedures, large fireballs can be seen emitted from these exhausts as fuel is ignited by each engine in turn. In order from port to starboard of the craft, each exhaust covers engines 1, 3, 2 and 4.
During startup, the main rear doors are closed, auxhilliary doors closed, then the engines are individually ignited and brought to idle in numerical order, checked, then the bow ramp is raised. The craft is then brought to full power whereupon the skirt is allowed to fully inflate and hovering is achieved. (James Rowson).